1. Field of the Invention
The present invention relates to a voltage converter, and more particularly, to a buck voltage converter that can generate an output signal with steady ripple.
2. Description of Related Art
FIG. 1A illustrates a conventional voltage converter 100. Referring to FIG. 1A, the voltage converter 100 includes a driver 110, a logic computing circuit 120, a comparator CMP1, transistors M1, M2, an inductor L1, a capacitor C1, and resistors R1 and R2. In the voltage converter 100, the comparator CMP1 compares a reference signal REF and a feedback signal VFB to obtain a comparison result, and the logic computing circuit 120 and the driver 110 sequentially generate driving signals DRV1 and DRV2 based on the comparison result. In addition, the transistor M1 receives a power supply voltage VIN.
In the voltage converter 100, when the comparator CMP1 determines that the feedback voltage VFB is lower than the reference voltage REF, the transistor M1 turns on for a period of time according to the driving signal DRV1, causing a step down output voltage VOUT to correspondingly increase. Thereafter, the transistor M1 turns off according to the driving signal DRV1, and the transistor M2 turns on according to the driving signal DRV2, causing the step down output voltage VOUT to decrease till the feedback voltage VFB becomes lower than the reference REF again. When the feedback voltage VFB becomes lower than the reference REF again, the transistor M1 turns on again for a period of time according to the driving signal DRV1.
FIG. 1B is a waveform diagram of the voltage converter 100 of FIG. 1B. Referring to FIG. 1B, when the effective series resistance (ESR) of the capacitor C1 is too low, a ripple portion of the feedback voltage VFB becomes extremely unconspicuous, which causes an unsteady state of the ripple phenomenon on the step down output voltage VOUT, thus reducing the quality of the step down output voltage VOUT.